glass-like appearance, often accompanied by prismatic colours, we can
hardly doubt that they thus escape the notice of pelagic birds and
other enemies. M. Giard is also convinced (1. Archives de Zoolog.
Exper. Oct. 1872, p. 563.) that the bright tints of certain sponges
and ascidians serve as a protection. Conspicuous colours are likewise
beneficial to many animals as a warning to their would-be devourers
that they are distasteful, or that they possess some special means of
defence; but this subject will be discussed more conveniently
hereafter.
We can, in our ignorance of most of the lowest animals, only say that
their bright tints result either from the chemical nature or the minute
structure of their tissues, independently of any benefit thus derived.
Hardly any colour is finer than that of arterial blood; but there is no
reason to suppose that the colour of the blood is in itself any
advantage; and though it adds to the beauty of the maiden s cheek, no
one will pretend that it has been acquired for this purpose. So again
with many animals, especially the lower ones, the bile is richly
coloured; thus, as I am informed by Mr. Hancock, the extreme beauty of
the Eolidae (naked sea-slugs) is chiefly due to the biliary glands
being seen through the translucent integuments this beauty being
probably of no service to these animals. The tints of the decaying
leaves in an American forest are described by every one as gorgeous;
yet no one supposes that these tints are of the least advantage to the
trees. Bearing in mind how many substances closely analogous to natural
organic compounds have been recently formed by chemists, and which
exhibit the most splendid colours, it would have been a strange fact if
substances similarly coloured had not often originated, independently
of any useful end thus gained, in the complex laboratory of living
organisms.
THE SUB-KINGDOM OF THE MOLLUSCA.
Throughout this great division of the animal kingdom, as far as I can
discover, secondary sexual characters, such as we are here considering,
never occur. Nor could they be expected in the three lowest classes,
namely, in the Ascidians, Polyzoa, and Brachiopods (constituting the
Molluscoida of some authors), for most of these animals are permanently
affixed to a support or have their sexes united in the same individual.
In the Lamellibranchiata, or bivalve shells, hermaphroditism is not
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